1. Field of the Invention
The present invention relates generally to a cover for a vehicle air bag. Particularly, the invention relates to a high safety type cover with optimal hardness of a main body thereof with a pleasing texture and density of an outer skin thereof and which will not fragment when breaking pressure is applied by an inflated air bag when the air bag is activated.
2. Description of the Prior Art
Conventionally, air bag cover arrangements are known utilizing a substantially soft material for covering the air bag such as polyurethane foam, for example. Such covers usually employ tear lines, or break lines arranged to allow the cover to open appropriately upon activation of the air bag.
Such conventional air bag covers are taught in Japanese Utility Model 56-48606, wherein a low density polyurethane foam may be utilized for a core portion, or main body, and a high density polyurethane foam utilized as a surface, or outer skin portion, the high and low density portions being joined together for forming as a single cover. Japanese Patent Application First Publication 63-199149 teaches using an insert of polyamide, polychloride vinyl, polyolefin, in a synthetic resin cover. Japanese Patent Application First Publication 63-232052 teaches a cover having an interior layer of polyamide and a jacket layer of polyurethane foam. However, using such techniques as described above in multi-layer arrangements, manufacturing thereof becomes complicated, also optimal adhesion is not present between the surfaces of the insert and the jacket layer.
In such conventional covers utilizing a variety of synthetic resins as set forth above, an adhesion strength between the core layer and the surface, or jacket layer, must be sufficient to prevent separation of the layers when the cover is in use installed in a vehicle. In order to increase the adhesion between the layers of such a cover, a plurality of small holes are formed in the core layer. However, in such a cover, when the air bag is inflated, tearing of the layers can diverge from a predetermined tear line. Further, in such conventional arrangements, when a core layer and a jacket layer are both formed of polyurethane foam, though sufficient adhesion may be present between the layer, both layers may bead up upon tearing and be introduced to a passenger compartment upon activation of the air bag.
Japanese Patent application First Publication 2-171364 discloses a cover wherein an insert or core is not utilized but the cover is formed integrally of a thermoplastic elastomer. Relief portions are defined in the elastomer for defining tear lines. However, since the relief portions for defining the tear lines must be formed in continuous fashion, an area in which the relief portions are formed becomes rather large. The cover is thus subject to cracking, or other damage as exposed to light and heat in the course of vehicle use. Further, such damage may degrade proper tearing along the break lines when the air bag is activated and the above mentioned drawback is present in that small pieces of the material may be widely spread in the passenger cabin upon opening of the cover.